Redis – Python AIOHTTP.web server multiprocessing load-balancer?

You’re right asyncio uses one CPU only. (one event loop uses one thread only and thus one CPU only)

Whether your whole project is network or CPU bound is something I can’t say.
You have to try.

You could use nginx or haproxy as load balancer.

You might even try to use no load balancer at all. I never tried this feature for load balancing, just as proof of concept for a fail-over system.
With new kernels multiple processes can listen to the same port (when using the SO_REUSEPORT option) and I guess it’s the kernel who would be doing a round robin.

Here a small link to an article comparing performance of a typical nginx configuration vs an nginx setup with the SO_REUSEPORT feature:

https://blog.cloudflare.com/the-sad-state-of-linux-socket-balancing/

It seems SO_REUSEPORT might distributes the CPU charge rather evenly, but might increase the variation of response times. Not sure this is relevant in your setup, but thought I let you know.

Added 2020-02-04:

My solution added 2019-12-09 works, but triggers a deprecation warning.

When having more time and time for testing it myself I will post the improved solution here. For the time being you can find it at AIOHTTP – Application.make_handler(…) is deprecated – Adding Multiprocessing

Added 2019-12-09:

Here a small example of an HTTP server, that can be started multiple times listening on the same socket.
The kernel would distribute the tasks. I never checked whether this is efficient or not though.

reuseport.py:

import asyncio
import os
import socket
import time
from aiohttp import web


def mk_socket(host="127.0.0.1", port=8000, reuseport=False):
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    if reuseport:
        SO_REUSEPORT = 15
        sock.setsockopt(socket.SOL_SOCKET, SO_REUSEPORT, 1)
    sock.bind((host, port))
    return sock

async def handle(request):
    name = request.match_info.get('name', "Anonymous")
    pid = os.getpid()
    text = "{:.2f}: Hello {}! Process {} is treating youn".format(
        time.time(), name, pid)
    time.sleep(0.5)  # intentionally blocking sleep to simulate CPU load
    return web.Response(text=text)

if __name__ == '__main__':
    host = "127.0.0.1"
    port=8000
    reuseport = True
    app = web.Application()
    sock = mk_socket(host, port, reuseport=reuseport)
    app.add_routes([web.get('/', handle),
                    web.get('/{name}', handle)])
    loop = asyncio.get_event_loop()
    coro = loop.create_server(
        protocol_factory=app.make_handler(),
        sock=sock,
        )
    srv = loop.run_until_complete(coro)
    loop.run_forever()

And one way to test it:

./reuseport.py & ./reuseport.py & 
sleep 2 # sleep a little so servers are up
for n in 1 2 3 4 5 6 7 8 ; do wget -q http://localhost:8000/$n -O - & done

The output might look like:

1575887410.91: Hello 1! Process 12635 is treating you
1575887410.91: Hello 2! Process 12633 is treating you
1575887411.42: Hello 5! Process 12633 is treating you
1575887410.92: Hello 7! Process 12634 is treating you
1575887411.42: Hello 6! Process 12634 is treating you
1575887411.92: Hello 4! Process 12634 is treating you
1575887412.42: Hello 3! Process 12634 is treating you
1575887412.92: Hello 8! Process 12634 is treating you